Chapter 2: Water and Plant Cells Flashcards
which resource is the most limiting for plants to grow and function?
water
why is water a limiting source?
- plants use water in huge amounts
- 97% of the water absorbed goes through transpiration
- only 2% of absorbed remains in the plant and 1% is used for biochemical reactions of photosynthesis
transpiration
the evaporation of water from the surface of leaves and stems
- dissipates heat energy, keeping leaves a few degrees cooler than the air (transpirational cooling)
why is water a supersolvent?
- because of hydrogen bonding and polar structure
- Water dissolves greater amounts of a wider variety of substances than do other related solvents
electronegative
Having the capacity to attract electrons and thus producing a slightly negative electric charge.
polarity
Property of some molecules, such as water, in which differences in the electronegativity of some atoms result in a partial negative charge at one end of the molecule and a partial positive charge at the other end.
- oxygen is more electronegative than hydrogen creating a partial positive and negative
what is water a good solvent for?
- ionic substances and for molecules such as sugars and proteins like (OH and NH2 groups)
- Hydrogen bonding between water molecules and ions, and between water and polar solutes, effectively decreases the electrostatic interaction (stabilizes the ions) between the charged substances and thereby increases their solubility
what are the distinctive properties of water?
Has a high specific heat capacity and high latent heat of vaporization
specific heat
Ratio of the heat capacity of a substance to the heat capacity of a reference substance, usually water.
why does water have a high specific heat?
- Hydrogen bonds act like rubber bands that absorb some of the energy from applied heat, leaving less energy available to increase motion
- Helps plants by buffering temp fluctuations
latent heat of vaporization
The energy needed to separate molecules from the liquid phase and move them into the gas phase at constant temperature.
Occurs during transpiration
For water at 25°C, the heat of vaporization is 44 kJ mol-1 - the highest value known for any Liquid.
cohesion
adhesion
The attraction of water to a solid phase such as a cell wall or glass surface, due primarily to the formation of hydrogen bonds.
capillarity
The movement of water for small distances up a glass capillary tube or within the cell wall, due to water’s cohesion, adhesion, and surface tension.
- Water is driven to climb walls of container by adhesion
- surface tension leads to minimizing of air-water interface
- cohesion pulls the rest of the water upwards
surface tension
the energy required to increase surface area
- influences shape and net force if surface is curved
what is tensile strength and does water have a high or low tensile strength?
The ability to resist a pulling force. Water has a high tensile strength due to cohesion.
hydrostatic pressure
Pressure generated by compression of water into a confined space. Measured in units called pascals (Pa) or. more conveniently, megapascals (MPa).
Pushing = a positive hydrostatic pressure
Pulling: a negative hydrostatic pressure
what do cellular processes depend on?
the transport of molecules both to the cell and away from it. (diffusion and osmosis)
diffusion
The movement of substances due to random thermal agitation from regions of high free energy to regions of low free energy (e.g., from high to low concentration).
osmosis
The net movement of water across a selectively permeable membrane toward the region of more negative water potential, (lower concentration of water).
- drives the movement of water into and out of cells
what does diffusion depend on?
- the net movement of molecules from regions of high concentrations to regions of low concentration
- The tendency for a system to evolve toward an even distribution of molecules can be understood as a consequence of the second law of thermodynamics, which is the spontaneous processes evolve in the direction of increasing entropy, or disorder.
flux density
The rate of transport of a substance s across a unit of area per unit of time.
diffusion coefficient
The proportionality constant that measures how easily a specific substance s moves through a particular medium. The diffusion coefficient is a characteristic of the substance and depends on both the medium and the temperature.
Fick’s first law
a substance will diffuse faster when the concentration gradient becomes steeper or when the diffusion coefficient is increased
Only applied to concentration gradients
- explained by simple random thermal agitation
when is diffusion most effective? why?
over short distances
As substances diffuse away from the starting point, the concentration gradient becomes less steep, solutes have moved forward and less are in the backwards
As it increases to 0, the net forces become smaller since it is approaching equilibrium
what is the average time required for a glucose molecule to diffuse across a cell of a diameter of 50 micrometers
2.5 s
the membranes of plants are selectively permeable, what does this mean?
They allow water and other small, uncharged substances to move across them more readily than larger solutes and charged substances
Solutes do not diffuse out of cell
- pass through a cell lipid bilayer and through aquaporins
chemical potential
a quantitative expression of the free energy associated with a substance
water potential
the chemical potential of water divided by the partial molal volume of water
A measure of the free energy of water per unit of volume
what are the three major factors that contribute to cell water potential?
Concentration, pressure, and gravity
Water potential is defined at ambient (room) temperature and standard atmospheric pressure. Reference height is normally at the base of the plant
solute (osmotic) potential
the effect of dissolved solutes on water potential
Reduce the free energy of water by diluting the water (caused by the entropy effect - the mixing of solutes and water increases the disorder or entropy of the system and lowers the free energy )
does the osmotic potential depend on the nature of the solute?
no
what is the solute potential formula?
-iRTc
pressure potential
Pressure potential represents the effect of hydrostatic pressure on the free energy of water.
Positive pressure raises the water potential, and negative pressures reduce it
tension
negative hydrostatic pressure and is important in moving water over long distances
what is influenced by water potential and its components?
cell growth, photosynthesis, and crop productivity
how can water potential be measured?
Determining the water potential can be done with psychrometers, which uses water’s heat of vaporization to measure the vapor pressure of water in equilibrium with the sample and the transfer of water vapor between the sample and a solution of a known solute potential
what water potential is typical of plant cells?
0 MPa or less
what is water potential at equilibrium?
water is equal everywhere, the water potential of the cell equals the water potential of the solution
is water flow active or passive? explain
Water flow across membranes is a passive process
Water moves in response to physical forces, toward regions of low water potential or low free energy
Water can only be forced to move across a semipermeable membrane without a water potential gradient is if it couple with the movement of solutes like sugars, amino acids, and other small molecules
what is the relationship between turgor pressure and cell volume?
cell elasticity
how does changes in cell volume affect changes in turgor pressure?
small changes in volume cause large changes in turgor pressure.
Because plant cells have fairly rigid walls, a change in cell water potential is generally accompanied by a large change in pressure potential, and no change to cell volume
Turgor pressure in most cells approaches zero as the relative cell volume decreases by 10-15%. The more rigid the cell, the less volume change occurs - think cactuses!
what happens when a cell experiences a change in water potential in its surroundings?
the movement of water across the plasma membrane decreases
hydraulic conductivity
how readily water can move across a membrane
Cells with large surface to volume ratios have high membrane hydraulic conductivity and vice versa
what facilitates the movement of water across plasma membranes?
aquaporins
define aquaporins and its relationship to water movement
integral membrane proteins that form channels across a membrane, many of which are selective for water. Such channels facilitate water movement across a membrane
Water diffuses faster in these channels than through osmosis
Aquaporins can affect the rate of water movement but not the direction or driving force
This allows plants to regulate the permeability of their plasma membranes
what physiological changes are affected during water loss?
halt plant growth and photosynthesis (cell expansion is most affected), shoot and leaf growth is halted although root growth is stimulated.
in addition - abscisic acid accumulation, solute accumulation, photosynthesis, stomatal conductance, protein synthesis, wall synthesis, and cell expansion
why do plants need to accumulate solutes during water loss
maintain turgor and volume
The plant may spend energy to accumulate solutes to maintain turgor pressure, invest in the growth of nonphotosynthetic organs such as roots to increase water uptake capacity, or build xylem conduits capable of withstanding large tensions.
What percentage of water does lettuce, wood, and seeds take up?
95%
37-75%
5-15%
what physiological changes occur at mild water stress? more intense?
- cell expansion, cell wall synthesis, and protein synthesis
- stomatal conductance, transpiration, accumulate abscisic acid and solutes
what is transpirational cost?
- the side effect of photosynthesis that exposes the interior of the plant to dry air
- in full sun, the leaf can exchange its total water content in 20 minutes
- on warm days 100% of water is exchanged every day
what does any flow require?
a driving force and a transport coefficient
what is the driving force of diffusion? the transport coefficient?
DF = concentration gradient TC = D = the property of the diffusing substance in the medium
what is diffusion used for?
- transpiration from leaves to air
- movement of solutes within cells
- movement of signaling molecules through plasmodesmata
what is the driving force of bulk flow? what is the transport coefficient?
DF = pressure gradient TC = hydraulic conductance
what is bulk flow used for?
- flow through the xylem and phloem
- flow through roots
- stem and leaf apoplast
- soil to roots
what is the driving force of osmosis? the transport coefficient?
DF = water potential gradient TC = membrane conductivity
